Field of Invention
The present invention relates to a supercapacitor. More particularly, the present invention relates to a supercapacitor using soft carbon as one of the electrode.
Description of Related Art
Electrochemical capacitor, also known as supercapacitor, is capable of stable charging or discharging at high speed. It is a high power energy storage component. Supercapacitor is well developed, and according to its storage mechanism, it is divided into three types. They are electrical double-layer capacitors (EDLCs), pseudo capacitor and asymmetric supercapacitor. EDLC uses the ion absorption and detaching in the electrolyte to generate power. Pseudo capacitor generates power from redox reaction. Asymmetric super capacitor uses at least two different types of mechanisms in one assembly to generate power. Energy density is the key to the development of electrochemical capacitor. According to energy density equation E=½CV2 (C is cell capacitance; V is cell voltage), there are two approaches to increase energy density. One way to increase energy density is to increase the cell capacitance, and the other way is to increase its cell voltage. A lot of time has been invested in manipulating the cell voltage so as to obtain an elevated energy density.
In order to increase cell voltage, a variety of organic electrolyte is used. Common organic electrolyte solvent includes propylene carbonate (PC), acetonitrile (AN), γ-butyrolactone (GBL) or the like. Solute may include, for example, tetraethylammonium tetrafluoroborate (TEABF4) and tetraethylammonium hexafluorophosphate (TEAPF6). It is found an electrolyte having the combination of PC and TEABF4 gives relatively high capacitance (dielectric coefficient).
In recent research, it is found that activated carbon could elevate capacitance to a great extent because of its porous structure and large surface area. The application of activated carbon in capacitor is heavily studied due to its unique electrochemical property. However, the porous structure retains a lot of water. In organic capacitor, water molecule facilitates decomposition of the organic electrolyte, leading to excessive gas production. Accumulative gas in the chamber results in surging pressure, and eventually the damage to the electrodes. Therefore, if activated carbon is used for negative and positive electrodes in a capacitor, the capacitor usually works under 2.5-2.7 V, or the device is prone to instable performance.
When using activated carbon as the positive electrode and graphene or Li-containing compound as the negative electrode, for example (Li4Ti5O12, LTO), the cell voltage should rise to 4.3 V. However, Li ions has low diffusion rate and poor conductivity, the power density is low, and therefore its empirical cell voltage is 3-4 V, which is lower than expected. Furthermore, in this type of Li-ion capacitor, solid-electrolyte interphase (SEI) is omitted for safety concern, and the minimum cell voltage in respect to Li/Li+ has to exceed 1.55 V. As a result, the actually remaining cell voltage is only approximately 2.45 V.
Therefore, there is an urgent call to provide a stable supercapacitor using carbon as the primary material and having a stable and high cell voltage.